1. Field of the Invention
The present invention relates to a dielectric resonator filter and, more particularly, to a dielectric resonator filter having low-loss characteristics.
2. Description of the Related Art
A conventional dielectric resonator filter is disclosed in, e.g., Japanese Unexamined Patent Publication (JP-A) No. 60-98702 (to be referred to as prior art 1 hereinafter).
In the dielectric resonator filter disclosed in prior art 1, a box-shaped metal case and a metal cover for covering the upper opening of the metal case constitute a rectangular-parallelopiped metal cavity. A plurality of support tables are arranged in the longitudinal direction of the case on the bottom surface in the metal case. A plurality of columnar dielectric resonators are arranged on the support tables. Input/output terminals having thin and long input/output probes extending in the metal case are arranged outside both the sides of the metal case. When one of the input/output terminals is an input terminal connected to the input probe, another one is an output terminal connected to the input probe. On the other hand, frequency adjustment metal screws are arranged at positions opposing the plurality of dielectric resonators of the metal cover. The intervals between the dielectric resonators and the metal screws are adjusted, so that the frequencies can be adjusted.
Since the input/output probes are electromagnetically coupled to the dielectric resonators, respectively, the input/output probes are arranged at positions each having a level which is almost equal to that of a center position of each dielectric resonator in height as positions at which optimum electromagnetic coupling can be achieved.
However, in a conventional dielectric resonator filter, input/output probes are attached to the central portions of one side of a rectangular metal case inside the metal case. Since the dimensions of the metal case are uniquely determined according to the distances between the input/output probes and the columnar dielectric resonators, the dielectric resonator filter cannot be easily reduced in dimension.
The dielectric resonator filter according to prior art 1 has an unnecessary resonance mode of the dielectric resonator and an unnecessary resonance mode determined by the shape and dimensions of the metal case including resonators. For this reason, a plurality of unnecessary resonance modes (HE, TM, and EH modes or the like) are disadvantageously generated in a band having a frequency which is 1.25 or more times a frequency f0 of a basic resonance mode (TEO01 xcex4 mode).
These unnecessary resonance modes can be suppressed by adding, e.g., low-pass filters or the like. For this reason, the system cannot be easily reduced in dimension.
It is an object of the present invention to provide a dielectric resonator filter which can be reduced in dimension.
It is another object of the present invention to provide a dielectric resonator filter which can be reduced in height and can be surface-mounted.
According to one aspect of the present invention, there is provides a dielectric resonator filter which includes a metal cavity. The metal cavity has a rectangular parallelopiped and in which at least one dielectric resonator is arranged between one pair of input/output probes. In the dielectric resonator filter, the input/output probes are attached to corner portions of the metal cavity.
According another aspect of the present invention, there is provided a dielectric resonator filter which includes a metal cavity. The metal cavity has a rectangular parallelopiped and in which at least one dielectric resonator is arranged between one pair of input/output probes. In the dielectric resonator filter, at least one electromagnetic wave abs orber is further attached to the in side of the metal cavity.